Yes, I did. It is not the fuse.
When I turn it on, I can hear some relais switch.
If an active source is connected, the display lights up and shows the input's data, however after a couple of seconds the displays goes off.
The little green leds stay on.
Roberto
This may be of no help but when I messed up the digital coax connection or had an issue with the ribbon cable the display would light, the LED's on the board would light but a message in the lower right of the LED display would come and go, about the only thing I could read is the word error. so with that in mind try this if you are comfortable with it. Remove the top cover of the dac, I will assume your dac is stock (not modded). Unscrew the coax cable, you may need a small pair of pliers to loosen, mark on end of the cable for orientation. Do you have a ohm meter? if so check continuity of this cable signal wire end to end and then shield end to end. If all good put back on. Now we will move over to the volume knob area of the inside, there you will see a 3 or 4 wire connector connected to the volume knob board, this wire runs to the LED board. Very carefully unplug this connector on the volume board and then plug back in making sure it has good contact, then do the same on the LED board be careful as it is kind of tight in there. (re-reading your post the most likely culprit are the steps below or the LED board is just flat out failing)
Once these 2 steps are done make sure the USB card is seated all the way down in its slot, just press on it. Now fire the dac up and see what happens. If the issue still occurs then unplug and we'll check the white ribbon cables that run from the digital board to the LED board and digital board to output board. These just pull straight up and out of the socket. There are 20 wire connectors in that ribbon, under a magnifying glass inspect them (the exposed silver) to make sure none are folded over and not making good clean contact with the socket, if so gently fold the bent one back down, maybe a spot of superglue to secure it back to the ribbon. Once satisfied all is good put the ribbon cable back the same way it was, the blue plastic at each end is to help gently slide the cable back into the socket. Do the same with the cable from digital to output board. Put that back and fire dac up again and hopefully the issue is now resolved and all is good.
There also is a tiny brownish ribbon cable running from the LED board to the actual display you can check this (it is on the volume knob side of the board), I cannot remember if it has a socket similar to the white cables or different, visually inspect it for damage.
Fingers crossed for good luck.
